What are the Impacts of Vegetation on Urban Air Pollution?
In terms of the interactions between vegetation and air pollutants in urban areas, vegetation and trees can both influence atmospheric composition of trace gases and enable dispersion and deposition of air pollutants, thus affecting the concentrations of pollutants that populations in urban areas are exposed to. A large amount of research has been undertaken to quantify the benefits of using vegetation to reduce concentrations in urban areas, however the research outcomes are variable and there is currently no clearly defined outcome to support the use of vegetation as a sole measure to improve air quality.
The Effect of Vegetation on Atmospheric Dispersion
The impact of trees on air flow and turbulence can influence the dispersal of pollutants across any area. The planting of trees locally (at a scale of 10 – 100m2 area) will influence dispersal characteristics which can lead to redistribution of pollution, but does not remove it. Completed studies show that where vegetation acts as a barrier close to a pollution source, e.g. a road link, concentrations immediately downwind of the planted area can be reduced typically by a factor of two, relative to the concentration if the vegetation were not in place. Conversely, however, concentrations can increase on the source side of the vegetation area. The extent to which the vegetation area influences dispersal is dependent on the planting design and specific species being used, with taller trees and shrubs being more effective that ground cover plantings.
The Effect of Vegetation on Deposition
Vegetation can increase surface areas for deposition of pollutants in an area. Deposition can take place through both dry and wet processes, with experimental studies based on field measurements and modelling. In terms of Particulate Matter (PM), the implementation of a planting scheme within an urban area resulted in concentration reductions in the range of 2% to 10%.
In terms of NO2, depositional rates are restricted by stomatal uptake and therefore are smallest in winter and during the night when urban NO2 concentrations are potentially at their highest. Vegetation is not considered to be a very effective sink for deposition of NOx species; as the deposition occurs during the daytime, and primarily in warmer months, there is little benefit for air quality for most of the periods where NO2 is a problem. As a consequence, the effectiveness of vegetation to reduce air pollution via deposition is considered minimal.
Vegetation’s Contribution to Emissions (Biogenic VOCs)
Biogenic Volatile Organic Compounds (BVOC) are currently present in small amounts in UK cities from existing vegetation emissions, which contribute to the formation of ambient O3. Increasing tree cover within urban areas has the potential to increase BVOC emissions, with the potential to increase ozone levels. This potential increase can be avoided however through selection of low BVOC emitting tree species and should be taken into consideration when selecting trees for planting in urban areas.
Overall, vegetation can contribute to a reduction in air pollution within urban areas, in particularly through their ability to influence the dispersal of pollutants across an area. However, they can only be a supporting solution to the air quality problems experienced on a city wide scale in terms of displacement and redistribution rather than removal of pollutants. Nonetheless, vegetation and green infrastructure in urban areas offers several other benefits, including improvements to biodiversity and wellbeing via improved mental health and recreational areas.
For more information on the impacts of vegetation on urban air pollution, please refer to the Air Quality Expert Group’s advice published in the report below.
Released: 2018 (PDF, 1.0MB, 40 pages)
Hosted on https://uk-air.defra.gov.uk